(i) Technical Field
The present invention relates to an image processing apparatus and a non-transitory computer readable medium.
(ii) Related Art
In an image forming apparatus using an electrophotographic process, control of calculating the number of pixels of an image to be printed and of replenishing a toner to a developing device in accordance with the calculated number of pixels is performed. This control is referred to as an image count dispense control (ICDC) type. When PDL data described in a page description language (PDL) is printed, the image forming apparatus generates image data to be printed by rasterizing the PDL data (converts the PDL data into data in a raster format) and performing a process such as a page fitting process, an enlargement and reduction, and a color space conversion to print the generated image data on a paper. The number of pixels of this image data is calculated and the calculated number of pixels is used for control of toner replenishing. The number of pixels obtained by using the ICDC process represents image density or image concentration of an image.
In the related art, toner replenishing control is performed based on an average of image density in the most recently printed pages of a predetermined number (for example, from tens of pages to hundreds of pages). However, this process is not suitable for print data in which image density is rapidly changed between pages. For example, when images having relatively low image density continue to a certain page and then pages having very high image density continue, toner replenishing may be insufficient at the first page of a group of the pages having high image density. In this case, if a toner supply amount is increased unexpectedly corresponding to a rapid increase in image density at a boundary between a low image density page and a high image density page, a fresh toner in the developing device is suddenly increased and thus unexpected color shift occurs.
As measures for this, it is considered that a buffer memory capable of holding several pages of print image data is prepared and sequentially generated print image data of each page is stored in the buffer memory in a first-in and first-out manner to be read and printed. In this process, a time difference from when print image data of a page is generated to when the print image data is printed on a paper may occur by the number of pages corresponding to the capacity of the buffer memory. Accordingly, image density of a page may be acquired earlier than the time difference from a point in time at which the page is printed and toner replenishing may be controlled such that an amount of a toner in a developing device is sufficient at the time when the page is actually printed. A control unit for toner replenishing detects a rapid change prior to the time difference from when a page having the rapidly changed image density is to be printed and causes a toner supply amount to be smoothed and changed to a certain extent during, for example, the time difference. Accordingly, an occurrence of unexpected color shift may be reduced or prevented.
There is an image forming system printing thousands of pages each minute among the recent high-end image forming systems. In such a high speed type, it is necessary to acquire image density of a page from tens of pages to hundreds of pages before the currently printed page in advance in order to obtain a time difference allowable for handling a delay of the toner supply control or rapid change in the toner supply amount. Accordingly, a buffer memory having a large enough capacity for holding print image data of tens of pages to hundreds of pages or more is necessary for applying the above-described process using the buffer memory. For example, a print speed of 2000 pages per minute corresponds to approximately 33 pages per second. If image density of print image data is necessary for control three seconds prior to a time of printing, a memory capacity large enough for buffering images of approximately 100 pages is necessary. For example, when high resolution printing such as 600 dpi and 1200 dpi is performed in a full color manner, the buffer memory having a capacity of tens of gigabytes to hundreds of gigabytes is required. Providing a memory having such a large capacity requires a large cost.
Even if the system has a buffer memory having a large capacity, a delay of time for which print image data of at least tens of pages to hundreds of pages is generated occurs in a time from when printing is instructed to when the first page is printed and output.
In the above description, toner supply control is described as a representative example. However, if the image density is acquired before the image is actually printed, many types of control may be performed in accordance with image density of an image among various types of control for other components (for example, temperature control of a fixing machine, correction of unevenness in a surface, and the like) in an image forming process of printing. There is also a problem in a case of such control of components, similarly to a case of toner supply control.